SWTPC 6800 weirdness

Brad H vintagecomputer at bettercomputing.net
Tue Sep 6 17:01:53 CDT 2016

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of william degnan
Sent: Tuesday, September 6, 2016 2:52 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: SWTPC 6800 weirdness

> >
> >
> >Brad,
> >You'll need to make electrical measurements, from the system checkout in
> >the manual.   You very possibly will have marginal components that need to
> >be replaced, but it's best to try to locate which is bad rather that 
> >to
> replace at random.
> >A000 is not the same place as 0100.    In the 64K space, they're quite
> >distant.
> >Eliminate all but the one RAM board, setting it to 0000.  Test that
> thoroughly, then add the next at the next RAM space beyond the first card.
> >Continue until you have enough RAM for a minimal Flex boot.  It 
> >should
> tell you in the version of Flex you're using how much that is (24K?)  
> It's hard to do everything at ?>the same time, break it down into chunks..
> >Bill
> Thanks Bill.  I've tried working with just single RAM boards, but like 
> I said, the only one that will work at all is this modified board.  I 
> have pics of it here:
> https://drive.google.com/folderview?id=0B4pq0-BHd2x6WVFiZHdyMHBlNW8&us
> p=
> sharing
> If I could understand better what it is set up to do, what address 
> spaces its occupying, I might be able to understand why my 16K DRC 
> boards don't work when I try to put them to $A000.  I'd prefer to work 
> with one of those boards first since the chips are socketed, and then 
> I could test the chips individually and be sure one whole board is good.
> I note in one of my pics there, the cap on that modified MP-M looks a 
> little tarnished on the outside...
>Getting a memory map of your system is an important step.  You need to know what memory addresses each board is attempting to use, so that there is no
>overlap.  Also remember that the ROM board has RAM on it too.   You would
>not want to map both boards to the same A000 space, but why do you need this at all?  What wants free RAM there?

>One important rule is that you don't want to overlap RAM.

>Can you get to a monitor prompt without any RAM installed other than that which is in the ROM board?


Based on what I've read, you *have* to have A000 if your CPU card has been modified for Flex 2.0, which I've verified mine has.  When mods on the older MP-A cards are done apparently it disables the onboard RAM, and that's where A000 would be.  I could reverse the mod but I'm not sure if I want to forgo FLEX use.  So yeah, according to SWTPC.com since that mod was done, I have to have a board at $A000.  However, setting either of my configurable ram boards to that space doesn't work.  The system will only boot with that weird MP-M in.  So there's more to it than that.. probably mods above and beyond.

I suppose it wouldn't be too bad to just reverse the Flex 2.0 mods and start there.  I'm doubtful if I'd ever use it and I could always reverse again if I do..

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